Reference may be made to previous works on different adsorbent materials such as alloys of molten metals like sodium/lead (U.S. Pat. No. 1,938,672), alkali or alkaline earth metal hydroxides (U.S. Pat. No. 4,003,823), alkali metal oxide based adsorbent (U.S. Pat. No. 4,007,109), potassium sulphide based adsorbent (U.S. Pat. No. 4,119,528), metal aluminate based adsorbents (U.S. Pat. No. 4,263,020), alkali metal alkoxide (U.S. Pat. No. 4,087,349). Various derivatives of hydrotalcite also referred to as LDH or layered double hydroxides exhibit high hydrogen sulfide sorption. Specific examples include Mg4.8 Al1.2(OH)12Cl1.2, Zn4Cr2(OH)12Cl2, Zn4Al2(OH)12Cl2, and may include numerous modified and unmodified synthetic and mineral analogs of these as described in U.S. Pat. Nos. 3,539,306, 3,796,792, 3,879,523, and 4454244, and as reviewed by Cavani et al. in Catalysis Today, Vol. 11, No. 2, pp. 173-301 (1991).
Some particularly active hydrogen sulfide sorbents are ZnSi2O5 gel, ZnFe2(OH)12Cl2, and the Fe containing clay like nontronite. Several Mg—Al hydrotalcites with crystallite size less than about 300 Angstroms also showed preference for hydrogen sulphide sorption (Bhattacharyya et al. Ind. Eng. Chem. Res. 1988, 27, 1356-1360). Pillared-smectites, kandites, LDHs (Layered Double Hydroxides) and silicic acids in which the layers are pillared by oxides of Fe, Cr, Ni, Co, Zn oxides in combination with alumina, as demonstrated by, but not limited to, U.S. Pat. Nos. 4,666,877, 5,326,734, 4,665,044/5 and Brindley et al, Clays and Clay Minerals, 26, 21 (1978) and Amer. Mineral, 64, 830 (1979) show very good hydrogen sulphide adsorption property.
Another method of desulphurization is use of molten dispersions of alkali metal alloys such as sodium/lead. Wherein sodium reacts with H2S to form Na2S (U.S. Pat. No. 1,938,672). However, this method has the disadvantage of difficult catalyst regenerability and the relatively low desulfurization ability etc.
Especially those hydrogen sulfide sorbents are preferred which are regenerable, representative of such sorbents are zeolitic materials, spinels, meso- and microporous transition metal oxides (U.S. Pat. No. 5,935,420). As described in the said patent adsorbents are prepared by impregnating metal salts over solid surfaces and finally converting the salts to oxides and making some extrudes out of these supported powders. The oxide and support combinations described are ZnO on alumina, CuO on silica, ZnO/CuO on kieselguhr and the like. Reagents useful for the regeneration of these types of hydrogen sulfide sorbents are air (oxygen), steam, hydrogen etc. This method of desulfurisation suffers from low reactivity of active adsorbent species present inside the body of the extruded pellets. Additionally, due to overheating there is formation of core sintering in such extruded pellets leading to further reactivity loss.
In all of these experiments adsorption of H2S is carried out at temperatures at around 500° C. and no special preparation of the particulate zinc alumina spinel or other adsorbents in nano dimension level is done thus effective adsorption in these systems is not maximum.
In all of these papers or patents the adsorption experiments were carried out by using either micron level powders or millimeter level pellets as adsorbents. In both of these forms, adsorbents had their own distinct disadvantages e.g. powder form of adsorbents give rise to high pressure drop and pellets on the other hand give rise to core sintering due to temperature gradient between the surface and the center. The coated honeycomb monolithic adsorbents used in the present invention do not possess such disadvantages. Indeed the pressure drop across the small unitary parallel passages of honeycomb type ceramic monoliths is two to three times smaller than the same in a collection of spherical pellets of equivalent area.
Reference may be made about the use of coated honey-comb type monolithic catalyst containing a synthetic mineral called hydrotalcite or layered double hydroxides (U.S. Pat. Nos. 3,796,792 and 3,879,523) as one of the coating component for the following U.S. Pat. Nos. 6,923,945 and 6,419,890. In the U.S. Pat. No. 6,923,945 hydrotalcite has been added with other inorganic components like alumina, zirconia, rare earth oxides and platinum metals etc. and applied as a coat to trap SOx so that actual ‘Three Way Catalyst’ containing the noble metals is not destroyed. In the second patent (U.S. Pat. No. 6,419,890) also hydrotalcites were added in the form of wash coatings over ceramic honeycombs to alleviate decrease of activity of ‘Three Way Catalyst’. It is to be noted here in none of these catalysts hydrotalcite particles are disaggregated to their individual sheet levels by taking course to any physical or chemical route or they were dispersed and coated as thin films over any favourably structured solid surface.
In the U.S. Pat. No. 1,938,672 alkali metal lumps of sodium, potassium or lithium were used in the fused state for desulphurization of hydrocarbon oils and lower boiling point hydrocarbons. This process thus uses costly metals and as well as removes sulphurous contaminants from liquid states only. In comparison to it the present process uses cheaper solid phase metal oxides in its exfoliated nano sheet form supported and coated as thin films over suitable structural supports.
In an another method reported in literature desulphurization was achieved through contact of sulphur bearing petroleum feedstocks with alkali or alkaline earth metal oxides/hydroxides in presence of hydrogen at high temperatures and pressure forming some alkali sulphides or hydrosulphides and finally the adsorbents were regenerated by reacting the adsorbents with steam at high temperature or oxidizing them to alkali metal sulphides in presence of activated carbon or in presence of magnesium oxide (U.S. Pat. No. 4,003,823). Here also to prepare the adsorbents, no particle particle electrostatic interaction between two layered or non layered materials was taken recourse to.
Another reported process for desulphurization and hydro conversion of heavy hydrocarbon feeds including various sulphur containing heavy petroleum oils is by contacting the feedstocks with potassium sulphide in a conversion zone maintained at elevated temperatures in presence of added hydrogen (U.S. Pat. No. 4,119,528). Similarly in another US patent (U.S. Pat. No. 4,007,109) a method has been described about desulphurization of petroleum feedstock by contacting the process steam with alkali metal oxides. In another US patent (U.S. Pat. No. 4,087,348) a method has been described about desulphurization of petroleum feedstock by contacting the process steam with alkali metal oxides, alkaline earth metal hydrides in presence of hydrogen at elevated temperatures. The alkaline earth metal sulfide salts can be regenerated to form alkaline earth metal hydrides or oxides.
In an another US patent (U.S. Pat. No. 4,087,349) a method has been described about simultaneous hydroconversion and desulphurization of petroleum feedstock by the reaction of feed with an alkali metal alkoxide in the presence of added hydrogen at elevated temperatures.
These processes thus are not basically for gaseous inorganic sulphur bearing compounds like SO2 or H2S formed during combustion or hydrogenation of hydrocarbons, as well as these processes need hydrogen as an additional reactant. The present process on the other hand can work in absence of hydrogen as well as in inert and air atmosphere.
Similarly, in an another US patent (U.S. Pat. No. 4,263,020) a method has been described about desulphurization of petroleum feedstock by contacting the process steam with particulate metal aluminates MAl2O4. The sulphur bearing compounds are adsorbed, desorbed and removed from the said particulate mass of metal alumina spinel by contacting and purging the same with a relatively clean gas stream suitably hydrogen, hydrogen containing gas or inert gas at elevated temperatures. Thus, this process also involves use of aggregated particulate adsorbents which possess the inherent problems of pressure drop, differential heat distribution between its surface and the core etc.
In an another US patent (U.S. Pat. No. 4,127,470) a method has been described about simultaneous hydroconversion and desulphurization of petroleum feedstock by the reaction of feed with binary combinations of reactive alkaline oxide/alkaline earth metal oxide or hydroxide, where the alkaline earth metal component is relatively reactive than alkaline metal oxide component. These desulfurization catalysts were then also supported over materials like coke, charcoal, alumina, silica, barium carbonate, barium sulphide, calcium oxide, calcium carbonate and like which provide a well dispersed supported reagent. The catalyst also claims that there is no relationship between different ratios of reactants in the adsorbent and may be varied considerably. In comparison to this process in the present process of patent application the desulphurisation adsorbents are coated over honeycomb monolithic supports having channeled geometry suitable for laminar flow of gases at high temperature in both oxidative and non-oxidative environment with minimum pressure drop in the system.
References may be made to patent application 3597DEL2012, wherein mixed metal oxidic nano sheets coated monolithic catalyst and preparation thereof is reported for the decomposition of toxic N2O. In the above referred invention a process for the preparation of nano-oxide coated catalysts useful for the treatment of toxic N2O gas by coating of composite materials containing LDHs over ceramic monolithic substrates is provided. The process combines reacting oxides and salts of metals in a known manner so as to prepare LDHs or mixed metal layered hydroxides such as Ni—Al, Mg—Al, Zn—Cr—Al type possessing positive layer charge, from which a stable gel is prepared by adding swellable clay having a negative charge e.g. montmorillonite, laponite, hectorite etc. in different LDH:clay ratio in an aqueous medium and homogenising the same with high speed homogeniser and ultrasonicator in a high intensity ultrasonic processor. The gel is then dip-coated over cordierite/mulite honey-comb monolithic supports at various dipping and withdrawal rates. The dip-coated monoliths are then dried and calcined at different temperatures to develop the alumino-silicate supported nano-oxide coats over honey-comb ceramic substrates for carrying out decomposition of N2O gas in a He flow in various flow rates at 400 to 600° C. temperature in a cylindrical quartz tube.
The use of hydrotalcite-clay composites to form supported oxides suitable for adsorption of toxic gases like H2S and SO2 have been patented by Goswamee et al. (Indian patent No 235052). In comparison to the same the present process is still a novel approach as it involves formation of thin coats of supported oxides by dip-coating them over ceramic honeycomb substrates, which apart from other advantages like increased activity due to nano dimensionality, supported projection, layered morphology, tunable metal ion composition etc. also would eventually lead to development of certain device to control air pollution by fixing it in the tail end of exhaust pipes of emission sources.